Study suggests willow trees grown in contaminated soil (phytoremediation) are more resistant to pests


Nature is amazing in its ability to defend and repair itself when harmed. A new study shows willow trees are able to turn on and off components of their DNA when planted in contaminated soil,?

Trees can decontaminate land, while protecting themselves from pests

Researchers from the University of Montreal have discovered that willow trees grown on contaminated land not only thrive, but they are attacked by fewer mites than those grown in uncontaminated soil. These results are further evidence of the advantages of using plants to decontaminate soil (phytoremediation).

Phytoremediation of contaminated land using willows

Willow trees are ideal for the phytoremediation of contaminated land, as they are quick-growing, and the wood has value as a biofuel. Other uses of willows include basketry and making cricket bats. The study in question used soil contaminated with C10 to C50 petroleum hydrocarbons, taken from the site of a former petroleum refinery. Similar uncontaminated soil from the site was used as a control. Purple willow (Salix purpurea) ?Fish Creek?, a very fast-growing variety, was grown in a greenhouse from cuttings, using either contaminated or uncontaminated soil.

RNA from willows in contaminated soil

After six months of growth, samples were taken from the buds, leaves, and stem of the saplings. RNA analysis was carried out, looking not just at the willow RNA, but that of any other species present. Apart from the willow RNA, the RNA from the two-spotted spidermite was present in large amounts from the trees grown in non-contaminated soil, but in much lower amounts in the trees grown in the contaminated soil. Of the RNA transcripts detected from the spidermite, 99% were found in higher abundance in the ?non-contaminated? samples.

RNA from various bacterial species was also found. Some bacterial species were found to be present in higher concentrations in the ?contaminated? samples, while others were more abundant in the ?non-contaminated? samples.

The results show that the response of the trees to contamination negatively effects the spidermite pests. The trees? response to the stress of contamination could be seen by changes to the RNA profile. RNA transcripts associated with the phenylpropanoid biosynthetic pathway were more in evidence in the ?contaminated? samples. This pathway produces various secondary metabolites, such as phenolic compounds, in response to stress. It is likely that these stress compounds were inhibiting the population of spidermites.

Willows as part of a ?meta-organism?

The various interacting species, plants, insects and bacteria, can be considered as a ?meta-organism?, or single entity. The ?meta-organism? undergoes changes in response to stress, and the changes in one species affect the others present.

Interestingly, a different study on mature willows on the site where the contaminated soil was obtained showed that another pest, the large willow aphid, seemed to prefer willows grown on non-contaminated soil.

Growth of willows in contaminated soil

In the greenhouse comparison studies, it was found that the overall growth of the trees in the contaminated soil was similar to that in the non-contaminated soil, showing that this variety of willow is suitable for use in remediation of soil contaminated by petroleum hydrocarbons. It is important that soil remediation does not negatively impact biomass production, since the sale for biofuel helps make the remediation more cost effective.

These results show that plants grown in contaminated soil can sometimes be less subject to pest attack. Willows are a useful species for the remediation of many contaminated soils, and can be less costly than other means of remediation. Professional advice should always be sought before any project involving the phytoremediation of contaminated land is undertaken.

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Source: Medlock, K. ?Trees thriving on contaminated land could help clean up humanity?s mess?, Inhabit website.
Gonzalez, E. et al, ?Meta-transcriptomics indicates biotic cross-tolerance in willow trees cultivated on petroleum hydrocarbon contaminated soil?, BMC Plant Biology, (2015), 15, 246.

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